Fixed geometry turbochargers can be designed to operate efficiently at a particular engine load and speed. However, when operated over a broad range of engine speed and load, the compressor and turbine components are forced to function off their design points and consequently suffer losses in efficiency that affects engine performance adversely. If the turbocharger is matched to an engine at the engine's rated speed, it will run considerably off its maximum efficiency where the engine is "torqued down" to low engine operating speeds. Conversely, if the turbocharger is matched to an engine's low speed range, the turbocharger will have a tendency to "overspeed" when the engine is operated at maximum speed and load.
To prevent overspeeding in turbochargers that have been matched to the low engine speed range, a waste gate is frequently used to bypass exhaust gas around the turbine to limit turbine speed over the high engine speed range. The waste gate, however, allows the escape of exhaust gas energy, which could be better utilized by the turbocharger turbine and results in a substantial loss in system efficiency.
A more efficient system generally known in the trade is one comprising variable geometry components in the turbocharger compressor, the turbocharger turbine, or both. The most common types are variable nozzle vanes ahead of the turbine wheel and/or variable diffuser vanes in the compressor component.
Variable nozzle vanes ahead of the turbine wheel are connected together so that the throat area of each nozzle passage can be reduced over the low engine speed range and increased as the engine speed approaches its maximum, so that the turbocharger speed is kept within a safe operating range. The positioning of the vanes must be precisely controlled by engine speed and load, and they must be freely movable in the hot exhaust gas environment with minimal leakage through clearance spaces.
The various movable devices that have been employed in the turbocharger turbine have been complicated, expensive, and subject to questionable durability. Consequently, they have met with limited commercial success.
A more practical approach to a variable device in the engine exhaust system was disclosed in U.S. Pat. No. 3,557,549 to Webster, assigned to Caterpillar Tractor Co., 1971. This system employs a flapper valve so positioned in a divided manifold system that it resides in a neutral position at high engine speed and load, but can be moved to a second position where it diverts all engine exhaust gas flow into one passage of a divided turbine casing at low engine speeds. This essentially doubles the flow of exhaust gas through the single turbine casing passage and maintains the turbocharger speed at higher levels than otherwise could be reached at low engine speeds. This device is much simpler than the complicated variable nozzle vane systems and does not require a precise control system for positioning.
The use of the flapper valve to divert exhaust gas allows the turbocharger to be matched efficiently to the higher engine speeds where the flapper is in a neutral position. When the engine is operated at low engine speeds, the diversion of full exhaust flow to the single turbine casing passage ahead of the turbine increases the turbocharger rotor speed to provide higher boost pressure to the engine cylinders, allowing the engine to produce more power and torque than otherwise could be obtained.
The increase in boost at low engine speeds produced by the diverted flapper valve might be great enough to cause the turbocharger compressor to operate in its surge or unstable area. In this case, the compressor must be rematched to move its surge line to lower air flow so that the engine operating points fall within its stable operating regime. However, this causes a movement of the compressor efficiency islands and choke area to lower flow and can result in lowering the compressor efficiency when the engine is operating at high speed and load.
A variable geometry compressor that can shift the performance map of the compressor to a lower or higher flow range is one solution to the problem of keeping the compressor out of surge at low engine speeds and still maintain high efficiency at high engine speeds. Variable diffuser vanes is one type of variable geometry compressor that could be employed, but the movable vanes cause significant mechanical complication internally in the construction of the turbocharger and must be precisely positioned by a rather elaborate control system.
A more practical type of variable geometry device is to employ movable pre-whirl vanes upstream of the compressor wheel to provide positive and negative pre-whirl to the air entering the inducer of the compressor wheel. Negative pre-whirl moves the compressor operating range to higher flow and usually improves compressor efficiency. Positive pre-whirl moves the compressor operating vane to lower flow and usually lowers compressor efficiency somewhat. However, since the maximum island of compressor efficiency is also moved to lower flow, the net effect of positive pre-whirl is to raise the level of efficiency available to the operating area of the engine.
It is thus advantageous to connect the movable flapper valve in the exhaust stream to the movable prewhirl vanes in the air stream by a mechanical linkage causing them to move in synchronization. With the flapper in neutral, the pre-whirl vanes are positioned to provide negative pre-whirl to the compressor, moving its flow range so that maximum efficiency is available in the high engine speed range. When the flapper is in the diverted position, the pre-whirl vanes are moved to the positive pre-whirl position, thereby moving the maximum compressor efficiency to the low engine speed range. A simple, hydraulic cylinder can be employed as a control means to move the mechanical linkage to either the high flow or low flow position by sensing the engine speed at which the transition is required to be made.
Both the flapper valve and the pre-whirl vanes are external from the turbocharger construction, resulting in much lower overall cost than other devices that must be built into the internal construction of the turbocharger.
The movement of the compressor flow range by utilizing positive and negative pre-whirl is more fully described in a paper published in the Proceedings of the Institute of Mechanical Engineers, Vol. 18943/75, titled "Experimental and Theoretical Performance of a Radial Flow Turbocharger Compressor with Inlet Pre-whirl," by Wallace, Whitfield and Atkey. It is also described in U.S. Pat. No. 5,025,629 to Woollenweber, June 1991.
At very low engine speed, for example, at low idle, there is insufficient exhaust gas energy to drive the turbocharger fast enough to produce significant levels of boost. Consequently, there is an appreciable lag time between opening of the engine throttle and when the turbocharger is running fast enough to produce enough boost pressure to eliminate smoke on acceleration, for example. Fuel control devices, such as rack limiters or aneroid controls, are employed to limit the amount of fuel delivered to the engine cylinders until the turbocharger is capable of delivering sufficient air to produce smoke-free combustion. These fuel limiting devices cause slower response to throttle opening and a sluggishness in engine and vehicle response.